Gas discharge lamps are well known in the art and their operation is described in FIGS. 1-4, to which reference is now made. FIG. 1 generally illustrates a gas discharge lamp and indicates that such a lamp includes a bulb 10, two electrodes 12 and 14, and gas 16 within the bulb 10. The lamp is controlled by a ballast 18 which includes an igniter 19 therein. Prior art gas discharge lamps are discussed in the OSRAM Metal Halide Lamps Technology and Application Handbook, July 1996, pp. 35-39 and 52.
To start the lamp, igniter 19 provides a spark, of typically 2-4 kV for a cold start and 20-40 kV for a hot start, between the two electrodes 12 and 14. The spark causes the electrode acting as the cathode, such as electrode 12, to emit electrons which ionizes the gas 16. The ionized gas then provides a low current path between the electrode 12 and the electrode 14, acting as the anode, thereby reducing the amount of voltage needed to close the circuit.
To ensures that the spark becomes established as a stable steady-state arc discharge, the spark must be of a high voltage (2-40 kV), the electrical energy of the spark must be high, the ballast must provide a quick current flow and the ballast must have an adequate open circuit voltage, typically of 250V.
The spark 20 is shown in the voltage-time graph of FIG. 2. Once ignition has occurred, the gas 16 is ionized and the voltage needed to maintain a current through the lamp drops to a low, operating voltage of about 20V, remaining there until the AC voltage direction changes. If the electrodes 12 and 14 are not sufficiently warm (i.e. they do not emit enough electrons), the ionization of the gas 16 cannot be maintained and the current path is broken. Accordingly, when the voltage changes direction, the gas must be reignited.
The reignition continues until the electrodes 12 and 14 are warm enough to maintain the ionization during the voltage direction change. This typically takes 10-100 cycles, where the length T1 of half of each cycle is typically on the order of 2.5 msec. Once this occurs, the operating voltage rises to the nominal operating voltage of the lamp which is typically between 50 and 130VAC and depends on the type of the lamp. FIG. 3 shows the cycles and the changing operating voltage over time.
The high power ignition pulses cause localized "hot spots" on the electrode, melting of the metal and sputtering of the electrodes 12 and 14 which erodes them. The sputtering blackens the inside walls of the bulb 10, thereby reducing the amount of light (as measured in lumens) that the lamp provides, a phenomenon known as "lumen degradation". Furthermore, the sputtering removes material from the electrodes, as shown in FIG. 4. FIG. 4 shows electrode 12 with a very uneven end 22. As more and more material is removed, the distance between the electrodes 12 and 14 is increased and, if the distance is too far, the spark does not successfully reach from one electrode to the other. Due to the two effects of sputtering and blackening, the lamp light output degrades dramatically and, eventually, the lamp fails.
Mechanisms are known for igniting the gas with a DC voltage and, once the gas is ignited, switching to AC operation. Since the voltage never changes direction, the gas 16 remains ionized. However, in such lamps, the current only attacks the electrode 14 acting as the anode, causing sputtering and warming up electrode 14 significantly more than electrode 12. The result is that portions of electrode 14 melt down, causing more severe damage than that seen with AC ignition.